Many physiological signals (e.g., sensory, hormonal and neurotransmitter signals) are transduced from extracellular to intracellular environments by cell surface receptors termed G-protein coupled receptors (GPCRs) (for a review, see Neer, 1995, Cell 80:249-257). Typically, GPCRs contain seven transmembrane domains. Putative GPCRs can be identified on the basis of sequence homology to known GPCRs.
GPCRs mediate signal transduction across a cell membrane upon the binding of a ligand to an extracellular portion of a GPCR. The intracellular portion of a GPCR interacts with a G-protein to modulate signal transduction from outside to inside a cell. A GPCR is therefore said to be "coupled" to a G-protein. G-proteins are composed of three polypeptide subunits: an .alpha. subunit, which binds and hydolyzes GTP, and a dimeric .beta..gamma. subunit. In the basal, inactive state, the G-protein exists as a heterotrimer of the .alpha. and .beta..gamma. subunits. When the G-protein is inactive, guanosine diphosphate (GDP) is associated with the .alpha. subunit of the G-protein. When a GPCR is bound and activated by a ligand, the GPCR binds to the G-protein heterotrimer and decreases the affinity of the G.alpha. subunit for GDP. In its active state, the G subunit exchanges GDP for guanine triphosphate (GTP) and active G.alpha. subunit disassociates from both the receptor and the dimeric .beta..gamma. subunit. The disassociated, active G.alpha. subunit transduces signals to effectors that are "downstream" in the G-protein signalling pathway within the cell. Eventually, the G-protein's endogenous GTPase activity returns active G subunit to its inactive state, in which it is associated with GDP and the dimeric .beta..gamma. subunit.
Numerous members of the heterotrimeric G-protein family have been cloned, including more than 20 genes encoding various G.alpha. subunits. The various G subunits have been categorized into four families, on the basis of amino acid sequences and functional homology. These four families are termed G.alpha..sub.s, G.alpha..sub.i, G.alpha..sub.q, and G.alpha..sub.12. Functionally, these four families differ with respect to the intracellular signaling pathways that they activate and the GPCR to which they couple.
For example, certain GPCRs normally couple with G.alpha..sub.s and, through G.alpha..sub.s, these GPCRs stimulate adenylyl cyclase activity. Other GPCRs normally couple with G.alpha..sub.q, and through G.alpha..sub.q, these GPCRs can activate phospholipase C (PLC), such as the .beta. isoform of phospholipase C (PLC.beta.) (Stermweis and Smrcka, 1992, Trends in Biochem. Sci. 17:502-506).
Certain G-proteins are considered "promiscuous" G-proteins because their G subunits allow them to couple with GPCRs that normally couple with G-proteins of other families. For example, two members of the G.alpha..sub.q family, human G.alpha..sub.16 and its murine homolog G.alpha..sub.15, have been shown in transient cell-based systems to possess promiscuous receptor coupling. Although G-proteins having these G subunits are promiscuous with respect to the GPCR with which they couple, these G-proteins retain the ability to couple with a specific downstream effector. In other words, regardless of which receptor is used to activate these G-proteins, the active promiscuous G subunit nonetheless activates PLC.beta..